Читать книгу Proust Was a Neuroscientist - Jonah Lehrer - Страница 19

Even as neuroscience began to reveal the brain’s surprisingly supple structure, other scientists were becoming entranced with an even more powerful deterministic principle: genetics. When James Watson and Francis Crick discovered the chemical structure of DNA, in 1953, they gave biology a molecule that seemed to explain life itself. Here was our source stripped bare, the incarnate reduced to some nucleic acids and weak hydrogen bonds. Watson and Crick recognized the handsome molecule the moment they assembled it out of their plastic atoms. What they had constructed was a double helix, a spiraling structure composed of two interwoven threads. The form of the double helix suggested how it might convey its genetic information. The same base pairs that held the helix together also represented its code, a hieroglyph consisting of four letters: A, T, C, and G.

Following Watson and Crick, scientists discovered how the primitive language of DNA spelled out the instructions for complex organisms. They summarized the idea in a simple epithet, the Central Dogma: DNA made RNA that made protein. Since we were merely elaborate sculptures of protein, biologists assumed that we were the sum of our DNA. Crick formulated the idea this way: “Once ‘information’ has passed into the protein [from the DNA,] it cannot get out again.” From the perspective of genetics, life became a neat causal chain, our organism ultimately reducible to its text, these wispy double helices afloat in the cellular nuclei. As Richard Dawkins declared in The Selfish Gene, “We are survival machines — robot vehicles blindly programmed to preserve the selfish molecules known as genes.”

The logical extension of this biological ideology was the Human Genome Project. Begun in 1990, the project was an attempt to decode the genetic narrative of our species. Every chromosome, gene, and base pair would be sequenced and understood. Our textual underpinnings would be stripped of their mystery, and our lack of freedom would finally be exposed. For the paltry sum of $2.7 billion, everything from cancer to schizophrenia would be eradicated.

That was the optimistic hypothesis. Nature, however, writes astonishingly complicated prose. If our DNA has a literary equivalent, it’s Finnegans Wake. As soon as the Human Genome Project began decoding our substrate, it was forced to question cherished assumptions of molecular biology. The first startling fact the project uncovered was the dizzying size of our genome. While we technically need only 90 million base pairs of DNA to encode the 100,000 different proteins in the human body, we actually have more than 3 billion base pairs. Most of this excess text is junk. In fact, more than 95 percent of human DNA is made up of what scientists call introns: vast tracts of repetitive, noncoding nonsense.

But by the time the Human Genome Project completed its epic decoding, the dividing line between genes and genetic filler had begun to blur. Biology could no longer even define what a gene was. The lovely simplicity of the Central Dogma collapsed under the complications of our genetic reality, in which genes are spliced, edited, methylated, and sometimes jump chromosomes (these are called epigenetic effects). Science had discovered that, like any work of literature, the human genome is a text in need of commentary, for what Eliot said of poetry is also true of DNA: “all meanings depend on the key of interpretation.”

What makes us human, and what makes each of us his or her own human, is not simply the genes that we have buried in our base pairs, but how our cells, in dialogue with our environment, feed back to our DNA, changing the way we read ourselves. Life is a dialectic. For example, the code sequence GTAAGT can be translated as instructions to insert the amino acid valine and serine; read as a spacer, a genetic pause that keeps other protein parts an appropriate distance from one another; or interpreted as a signal to cut the transcript at that point. Our human DNA is defined by its multiplicity of possible meanings; it is a code that requires context. This is why we can share 42 percent of our genome with an insect and 98.7 percent with a chimpanzee and yet still be so completely different from both.

By demonstrating the limits of genetic determinism, the Human Genome Project ended up becoming an ironic affirmation of our individuality. By failing to explain us, the project showed that humanity is not simply a text. It forced molecular biology to focus on how our genes interact with the real world. Our nature, it turns out, is endlessly modified by our nurture. This uncharted area is where the questions get interesting (and inextricably difficult).

Take the human mind. If its fissured cortex — an object that is generally regarded as the most complicated creation in the known universe — were genetically programmed, then it should have many more genes than, say, the mouse brain. But this isn’t the case. In fact, the mouse brain contains roughly the same number of genes as the human brain. After decoding the genomes of numerous species, scientists have found that there is little correlation between genome size and brain complexity. (Several species of amoeba have much larger genomes than humans.) This strongly suggests that the human brain does not develop in accordance with a strict genetic program that specifies its design.

But if DNA doesn’t determine the human brain, then what does? The easy answer is: nothing. Although genes are responsible for the gross anatomy of the brain, our plastic neurons are designed to adapt to our experiences. Like the immune system, which alters itself in response to the pathogens it actually encounters (we do not have the B cells of our parents), the brain is constantly adapting to the particular conditions of life. This is why blind people can use their visual cortex to read Braille, and why the deaf can process sign language in their auditory cortex. Lose a finger and, thanks to neural plasticity, your other fingers will take over its brain space. In one particularly audacious experiment, the neuroscientist Mriganka Sur literally rewired the mind of a ferret, so that the information from its retina was plugged into its auditory cortex. To Sur’s astonishment, the ferrets could still see. Furthermore, their auditory cortex now resembled the typical ferret visual cortex, complete with spatial maps and neurons tuned to detect slants of light. Michael Merzenich, one of the founders of the plasticity field, called this experiment “the most compelling demonstration you could have that experience shapes the brain.” As Eliot always maintained, the mind is defined by its malleability.*

This is the triumph of our DNA: it makes us without determining us. The invention of neural plasticity, which is encoded by the genome, lets each of us transcend our genome. We emerge, characterlike, from the vague alphabet of our text. Of course, to accept the freedom inherent in the human brain — to know that the individual is not genetically predestined — is also to accept the fact that we have no single solutions. Every day each one of us is given the gift of new neurons and plastic cortical cells; only we can decide what our brains will become.

The best metaphor for our DNA is literature. Like all classic literary texts, our genome is defined not by the certainty of its meaning, but by its linguistic instability, its ability to encourage a multiplicity of interpretations. What makes a novel or poem immortal is its innate complexity, the way every reader discovers in the same words a different story. For example, many readers find the ending of Middlemarch, in which Dorothea elopes with Will, to be a traditional happy ending, in which marriage triumphs over evil. However, some readers — such as Virginia Woolf — see Dorothea’s inability to live alone as a turn of plot “more melancholy than tragedy.” The same book manages to inspire two completely different conclusions. But there is no right interpretation. Everyone is free to find his or her own meaning in the novel. Our genome works the same way. Life imitates art.